Process heat boilers are commonly used with many types of industrial heat sources to extract heat from process gases of an industrial process. It may be necessary to extract heat from the process gas to cause a component thereof to condense, or it may be advantageous to extract heat from the process gas and use that heat in another process or even to provide heat for the industrial facility.
Generally speaking, a process heat boiler includes a plurality of metal boiler tubes supported by opposed metal tube sheets. The tube sheets define a vessel for holding water or some other form of heat transfer medium. Hot process gas passes through the boiler tubes arranged in the inlet tube sheet and heat is extracted therefrom via heat transfer from the hot gas to the water contained within the confines of the tube sheets. In higher temperature applications this type of process heat boiler requires a refractory face on the tubesheet exposed to the high temperatures. The current technology for providing this refractory face (hotface) is to employ ceramic hexagonal (hex) or square head ferrules (see, e.g., U.S. Pat. No. 5,647,432, owned by Blasch Precision Ceramics, Inc.).
With the current ceramic hex ferrule or square head ferrule technology, the outer shape of the head portion of the ferrule enables the mating of a plurality of adjacent ferrules in an array to cooperatively form a substantially gas-tight refractory barrier wall. The outer peripheral dimension of each ferrule is selected such that the ferrules are separated from one another when the industrial heat source is inoperative, and are abutted/mated at respective outer peripheral surfaces when the industrial heat source is operative, whereby a substantially gas-tight seal is formed between the entirety of respective outer peripheral surfaces of adjacent ferrules. The head portions of the ferrules have sufficient axial length to perform the function of the castable refractory wall, that is, to shield the inlet tube sheet from the process gas. The tube portions of the ferrules shield the inlet ends of the boiler tubes.
FIGS. 1A to 1C show a current technology ceramic hex ferrule 300 to be installed in a tube sheet 41 in connection with a process heat boiler, where FIG. 1C clearly shows the head gasket material 350 wrapped in the peripheral head groove 311. FIG. 1D shows a complete tubesheet array 40 including current technology ceramic hex ferrules 300 installed in the openings 42 of the tubesheet 41 to define the array.
The current ceramic hex ferrule 300, or square head ferrule technology, has proven to work well in most applications due, in large part, to the expansion gap provided between the individual adjacent ferrule heads, which eliminates mechanical loading on the heads due to accumulated thermal expansion across the tubesheet, and provides effective containment of the head gasket material seated in the recess area along the head periphery.
For example, as shown in FIGS. 1A and 1B, the standard hex ferrule 300 includes a hexagonally shaped head portion 301 and a substantially cylindrical stem portion 320 extending therefrom. The stem portion 320 also includes a tapered portion 322 located proximate the end thereof closest to the head portion 301, which transitions to the cylindrical part 324 at transition point 325.
The head portion 301 includes an upper surface 302 that includes a tapered (radiused) portion 304 curving downwardly from the peripheral edge of the opening 308 toward the outer peripheral edge of the upper surface 302. The tapered portion 304 of the upper surface 302 of the head portion 301 of the hex ferule 300 serves to reduce disruption and detachment of gas flow thereby reducing the overall pressure loss.
The head portion 301 of the hex ferrule 300 also includes a central annular recess 311 in the outer surface 307 thereof, which is suitable for the provision of gasket material 350, such as fiber wrap therein (as shown in FIG. 1C).
However, this technology can be problematic in situations where the tubesheet to be protected exhibits a high variability in the tube to tube pitch (e.g., tube center to tube center distances). This can be seen in older tubesheets that were not originally manufactured to the close tolerances that are standard today, or it can occur on tubesheets that have experienced some degree of maintenance and repair. This high variability (i.e., >0.03″) can be troublesome with respect to the installation of precision ceramic ferrules, where the gap for thermal expansion between the ferrule heads is small, ranging, depending on boiler design and temperature, from 0.020″ to over 0.060″, for example. In some instances, the ferrules need to be cut to fit. In other instances, the gap for thermal expansion may be too small, causing stress and reliability concerns.
In order to accommodate for excessive variability of tube to tube pitch in a tubesheet, it is necessary to increase the design gap between each ferrule head at assembly. This is accomplished by reducing the flat to flat distance across each individual ferrule head. The flat to flat distance is understood to be the distance between any pair of edges of the hexagonally shaped head portion on a single ferrule. An example of this type of array 410 including slightly modified hex ferrules 300A can be seen in FIG. 2. The hex ferrules 300A have essentially the same structures as the hex ferrules 300, with the exception that the size of the hexagonally shaped head portions are slightly smaller so as to reduce the overall flat to flat distance, as discussed above. Other aspects of the hex ferrules 300A remain consistent with those described in connection with the hex ferrules 300, and repeat descriptions are omitted. The hex ferrules 300A also include gasket material in the annular recess thereof in the same manner as shown in FIG. 1C, for example.
As shown in FIG. 2, the array 410 includes a tubesheet 411 having an irregular pitch between the openings 412 therein, and the hex ferrules 300A installed therein provide additional gaps between adjacent head portions to accommodate thermal expansion and variability in tube to tube pitch. FIG. 3 depicts a partial sectional view of some of these ferrules 300A showing the additional gap at the hot face (top of the ferrule) in relation to the head recessed area that will contain the head gasket material. However, a problem arises in that this additional gap needed to accommodate for the thermal expansion variability also leaves a larger path for process gas to impinge on the head gasket material and can provide an avenue for gasket material to slide out from between the heads.